Multiple-pulse sequence pulses

The next step, therefore, is to address the question how is it possible to take advantage of the many additional available parameters pulse shaping, multiple pulse sequences, etc—m general an E(t) with arbitrary complexity—to maximize and perhaps obtain perfect selectivity Posing the problem mathematically, one seeks to maximize... 

Some modifications of the C7 sequence are based on a variation of the basic pulse unit. The POST-C7 sequence uses a basic element of (n/2)v(2n)v+]z( in/2)vt [76]. The CMR7 sequence uses two different basic elements which are combined in a super cycle [79]. Alternative schemes have been proposed that reduce these rf field requirements, matching the rf field amplitude to five times the spinning frequency, which may in some instances be preferable [80]. There are also newer multiple-pulse sequences based on the same symmetry principles as the C7 sequence which have significantly lower rf field requirements [81-84],... 

The influence of the homonuclear magnetic dipole-dipole interaction on can be reduced either by an increase of the sample spinning frequency, Vjot, (Eq. (20)) or by the application of a multiple-pulse sequence causing an additional averaging of this interaction (combined rotation and multiple-pulse spectroscopy, CRAMPS 19-21 ). With today s instruments, sample spinning frequencies of up to 40 kHz can be reached using MAS NMR rotors with an outer diameter of 2.0 mm. 

The acronym COSY stands for Correlated SpectroscopV and this technique is widely used to determine all of the coupling interactions in a single experiment. This proves to be more efficient than the decoupling experiment in which each signal is irradiated in turn to determine its coupling partners. COSY involves a multiple pulse sequence (which we do not need to know anything about in order to use the technique) and is an example of two-dimensional (2D) spectroscopy. 

Optical Multiple Pulse Sequences for Multiphoton Selective Excitation and Enhancement of Forbidden Transitions, W. S. Warren and A. H. Zewail, J. Chem. Phys. 78 (11), 3583 (1983). 

S. Mukamel In general, multipulse experiments depend on a multitime correlation function of the dipole operator [1], The term x(n) depends on a combination of n + 1 time correlation functions. Their behavior for large n will depend on the model. In some cases (e.g., the accumulated photon echo used by Wiersma) the multiple-pulse sequence is simply used to accumulate a large signal and the higher... 

A number of different multiple pulse sequences (8-, 24- and 52-pulse sequences) have also been introduced in order to obtain better resolution or line narrowing, i.e. to affect the first- and second-order terms in the average Hamiltonian. Since pulse imperfections are the major source of resolution limitations, these composite pulse sequences are designed with corresponding symmetry properties which allows the canceling of specific rf pulse imperfections. 

Undesired homonuclear spin interactions can be also suppressed using suitable multiple-pulse sequences while still exploiting the information content provided by interactions that are not affected. Using a combination of MAS and pulse decoupling it is even possible to reintroduce parts of an interaction that would be averaged out by one of the manipulation techniques alone ( recoupling ) [11]. This high flexibility of solid-state NMR enables one to fully exploit the rich information content provided by the spin interactions. It becomes particularly powerful if such experiments are combined to multidimensional NMR techniques as discussed in Section 14.3. 

When the simple one-pulse experiment is again considered, there is only one time factor (or variable) that affects the spectrum, namely the acquisition time, f2. We now consider a multiple-pulse sequence in which the equilibration period is followed by two pulses with an intervening time interval, the final pulse being the irll acquisition pulse. Thus, we have inserted an evolution period between the pulses. If we now vary this evolution time interval (f3) over many different experiments and collect the resulting FIDs into one overall experiment, we have the basis of a 2-D experiment. Sequential Fourier transformation of these FIDs yields a set of spectra whose peak intensities vary sinusoidally. This first series of Fourier transformations result in the second frequency axis, v2, derived from the acquisition time, r2, of each FID. The data are now turned by 90°, and a second Fourier transformation is carried out at right angles to the first series of transformations. This second series of Fourier transformations result in the first frequency axis, iq, a function of the evolution time, f1 which you recall was changed (i.e., incremented) in the pulse sequence for each successive FID. 

The major breakthroughs, however, have come from the use of high magnetic fields and further from the use of different multiple pulse sequences to manipulate the nuclear spins in order to generate more and more information time domain NMR spectroscopy, that is used to probe molecular dynamics in solutions. The latter made it also possible to "edit" sub-spectra and to develop different two-dimensional (2D) techniques, where correlation between different NMR parameters can be made in the experiment (e.g. SH versus 813c, see later). Solid state NMR spectroscopy is used to determine the molecular structure of solids. 

The strong dipolar coupling (1) can be greatly reduced by application of high power decoupling (HPD) techniques in case of H NMR multiple pulse sequences are necessary. 

This 2D correlation method uses a multiple-pulse sequence to suppress resonance offsets and promote polarisation transfer driven only by the /-coupling while simultaneously avoiding the recoupling of dipolar interactions. A much... 

Despite numerous applications, conventional CRAMPS still remains one of the most demanding solid state NMR experiments as it requires the use of specially prepared spherical samples to minimise radiofrequency inhomogeneity effects and the careful calibration and setting of pulse widths and phases. Further modifications of the experiment that do not require the complicated and extended set-up procedures have been suggested recently. These are known as rotor-synchronised CRAMPS, which combines a new multiple pulse sequence [21], and its modification which uses a standard WHH-4 sequence at ultrafast MAS frequencies (e.g. 35 kHz) [22]. 

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